A. H. OLARK : THE CRINOIDS OF THE INDIAN OCEAN. 17 



have progressed to the extreme point indicated (or perhaps slightly further) and 

 then have been subjected to some change in environmental conditions which has 

 served as a stimulus and sent a greater or lesser part of the fauna some distance 

 back along the phylogenetic faunal path. 



The fauna of the Bering Sea appears to be a very young fauna. The crinoids 

 of the shallower waters here are abundant, but all the species, which are very 

 variable, belong to the genus Solanometra , an intrusion from the antarctic regions. 

 Of the other echinoderms the starfish present a wealth of forms, maddening to the 

 systematist ; the number of varieties and of incipient and valid species produced 

 from the Ctenodiscus , Hipjxtsteria, Solaster, Henricia, and other types is almost 

 incredible. Conditions are the same among the echinoids and among the ophiu- 

 roids, and apparently among many, if not most, other animal groups as well. 

 Yet with all this variability there is but a slight tendency to produce pathological, 

 defective, or unbalanced types, types which depart widely from the generic mean 

 as calculated from a study of the same genera in other areas. 



So far as the crinoids are concerned the antarctic region is very young ; here 

 we have Solanometra and Prouiachocrinus (the latter merely differing from 

 the former in the doubling of all the radials) each with several very variable 

 species, though none so variable as the Bering Sea representatives of the same 

 group. 



The crinoid fauna of southern Japan might be considered as an adolescent 

 fauna: here we find many genera including several species, each very stable and 

 showing comparatively little variation, such as Catoptometra, Comanthus , Dichro- 

 metra, Parametra, Pectinometra^ Thaumatometra and. Pentametrocrimis , while 

 Erythrometra , Nanometra, Calomefra, Carpenterocrinus and Phrynocrinus are not 

 known elsewhere though the two last, being from deep water, probably occur to 

 the southward. 



The West Indian crinoid fauna appears to be approximately a mature fauna. 

 It contains a number of pecuUar genera, while almost all of the East Indian 

 genera which occur here have become more or less differentiated from the original 

 stock forming new genera parallel to the original East Indian types. 



The Australian fauna is a perfect example of a senescent fauna. It includes 

 about fifty-five species, nearly all of which are remarkable for the grotesque ex- 

 aggeration of their specific characters. Even in certain wide-ranging forms, such 

 as ComatuJa solans or C pectinata, Australian specimens have their characters 

 greatly accentuated over those from other regions. 



A rejuvenated fauna is indicated by the crinoids about the shores of the large 

 East Indian Islands ; some of these approach, in the exaggeration of their specific 

 characters, the Australian species, while others are very generalized with several 

 closely related forms. The crinoid fauna of western Europe is also a rejuvenated 

 fauna ; in the case of each of the two component genera the more primitive 

 species are found in the Mediterranean Sea. 



It is possible to analyze a fauna on the basis of a single character in a group. 



